Shukti Chakravarti

Bio: Shukti Chakravarti is an academic researcher from Johns Hopkins University. The author has contributed to research in topics: Lumican & Cornea. The author has an hindex of 40, co-authored 83 publications receiving 6377 citations. Previous affiliations of Shukti Chakravarti include Johns Hopkins University School of Medicine & Case Western Reserve University.

Lumican Regulates Collagen Fibril Assembly: Skin Fragility and Corneal Opacity in the Absence of Lumican

Abstract: Lumican, a prototypic leucine-rich proteoglycan with keratan sulfate side chains, is a major component of the cornea, dermal, and muscle connective tissues. Mice homozygous for a null mutation in lumican display skin laxity and fragility resembling certain types of Ehlers-Danlos syndrome. In addition, the mutant mice develop bilateral corneal opacification. The underlying connective tissue defect in the homozygous mutants is deregulated growth of collagen fibrils with a significant proportion of abnormally thick collagen fibrils in the skin and cornea as indicated by transmission electron microscopy. A highly organized and regularly spaced collagen fibril matrix typical of the normal cornea is also missing in these mutant mice. This study establishes a crucial role for lumican in the regulation of collagen assembly into fibrils in various connective tissues. Most importantly, these results provide a definitive link between a necessity for lumican in the development of a highly organized collagenous matrix and corneal transparency.

Ulcerative colitis and Crohn’s disease: distinctive gene expression profiles and novel susceptibility candidate genes

Abstract: To elucidate the biological dysregulation underlying two forms of inflammatory bowel disease (IBD), ulcerative colitis (UC) and Crohn's disease (CD), we examined global gene expression profiles of inflamed colonic tissue using DNA microarrays. Our results identified several genes with altered expression not previously linked to IBD. In addition to the expected upregulation of various cytokine and chemokine genes, novel immune function-related genes such as IGHG3, IGLL2 and CD74, inflammation-related lipocalins HNL and NGAL, and proliferation-related GRO genes were over-expressed in UC. Certain cancer-related genes such as DD96, DRAL and MXI1 were differentially expressed only in UC. Other genes over-expressed in both UC and CD included the REG gene family and the calcium-binding S100 protein genes S100A9 and S100P. The natural antimicrobial defensin DEFA5 and DEFA6 genes were particularly over-expressed in CD. Overall, significant differences in the expression profiles of 170 genes identified UC and CD as distinct molecular entities. The genomic map locations of the dysregulated genes may identify novel candidates for UC and CD genetic susceptibility.

Differential expression of lumican and fibromodulin regulate collagen fibrillogenesis in developing mouse tendons.

Abstract: Collagen fibrillogenesis is finely regulated during development of tissue-specific extracellular matrices. The role(s) of a leucine-rich repeat protein subfamily in the regulation of fibrillogenesis during tendon development were defined. Lumican-, fibromodulin-, and double-deficient mice demonstrated disruptions in fibrillogenesis. With development, the amount of lumican decreases to barely detectable levels while fibromodulin increases significantly, and these changing patterns may regulate this process. Electron microscopic analysis demonstrated structural abnormalities in the fibrils and alterations in the progression through different assembly steps. In lumican-deficient tendons, alterations were observed early and the mature tendon was nearly normal. Fibromodulin-deficient tendons were comparable with the lumican-null in early developmental periods and acquired a severe phenotype by maturation. The double-deficient mice had a phenotype that was additive early and comparable with the fibromodulin-deficient mice at maturation. Therefore, lumican and fibromodulin both influence initial assembly of intermediates and the entry into fibril growth, while fibromodulin facilitates the progression through growth steps leading to mature fibrils. The observed increased ratio of fibromodulin to lumican and a competition for the same binding site could mediate these transitions. These studies indicate that lumican and fibromodulin have different developmental stage and leucine-rich repeat protein specific functions in the regulation of fibrillogenesis.

Corneal Opacity in Lumican-Null Mice: Defects in Collagen Fibril Structure and Packing in the Posterior Stroma

Abstract: It has long been recognized that collagen architecture of the corneal stroma is crucially important in the ultimate transparency of the cornea.1 Collagen fibrils in the stroma are maintained in the range of 20 to 40 nm and organized into a highly ordered, latticelike configuration. The highly ordered architecture of the corneal stroma is affected by multiple factors. Recent studies of several types of hereditary corneal dystrophies elucidated abnormal collagen fibril architecture of the corneal stroma. For example, corneal opacification is a clinical feature of Scheie’s syndrome or mucopolysaccharidosis (MPS) type I, a lysosomal storage disorder with an iduronidase A deficiency.2 In addition to featuring granular deposits, transmission electron microscopy of MPS I–affected corneas revealed the presence of thicker collagen fibrils and localized disorganization of the matrix.3,4 Macular corneal dystrophy, with deficiencies in keratan sulfate (KS) biosynthesis, also causes clouding of the cornea and similar disruptions in stromal fibril structure and organization.5–8 In both cases, altered proteoglycan synthesis and composition are to be expected. Recently, a mouse model for corneal dystrophy was developed by targeted disruption of the lumican gene (lumtm1sc/lumtm1sc).9 The mutant mice had cloudy corneas and stromal collagen fibrils with increased diameter and altered structure. Lumican is a member of the leucine-rich proteoglycan (LRP) family.10 It is a major keratan sulfate proteoglycan of the corneal stroma as well as other collagenous extracellular matrices (skin, cardiac valves, cartilage, and bone).11 Other LRP members include decorin, fibromodulin, biglycan, keratocan, osteoglycin, and epiphycan.12 Decorin, a chondroitin sulfate (CS) proteoglycan widely expressed during mouse embryonic development, is also a major component of the corneal stroma.13 Previous studies have shown that the core proteins of lumican, decorin, and other LRPs from tendons can delay spontaneous collagen fibril formation and inhibit the lateral growth of fibrils in fibrillogenesis assays in vitro.14–16 Also, the abnormal lateral growth of isolated corneal fibrils stripped of their surface-associated macromolecules is prevented by the corneal proteoglycans.17 Recent gene-targeting studies of LRPs suggest a similar role for these proteoglycans in vivo. Thus, absence of lumican in our lumtm1sc/lumtm1sc mouse model of corneal dystrophy affected collagen architecture of the cornea and skin with consequent corneal opacity and reduced dermal biomechanical tensile strength. In addition to lumican, gene-targeted null mutations in decorin and fibromodulin also led to abnormal collagen fibril architecture in skin and tendons.18,19 However, to date only the lumican-deficient mice have demonstrated a corneal phenotype. The purpose of the present study was to assess corneal opacification in the lumtm1sc/lumtm1sc mice and define its source in the corneal stroma by in vivo confocal microscopy. Parallel analyses of collagen fibril structure, fibril packing, and organization in the lumican-deficient and wild-type control mice and lumican expression in the mature normal cornea indicate that lumican serves a key role in the establishment and maintenance of corneal transparency.

Myofibroblasts and mechano-regulation of connective tissue remodelling

Abstract: During the past 20 years, it has become generally accepted that the modulation of fibroblastic cells towards the myofibroblastic phenotype, with acquisition of specialized contractile features, is essential for connective-tissue remodelling during normal and pathological wound healing. Yet the myofibroblast still remains one of the most enigmatic of cells, not least owing to its transient appearance in association with connective-tissue injury and to the difficulties in establishing its role in the production of tissue contracture. It is clear that our understanding of the myofibroblast its origins, functions and molecular regulation will have a profound influence on the future effectiveness not only of tissue engineering but also of regenerative medicine generally.

Matrix Proteoglycans: From Molecular Design to Cellular Function

Abstract: The proteoglycan superfamily now contains more than 30 full-time molecules that fulfill a variety of biological functions. Proteoglycans act as tissue organizers, influence cell growth and the maturation of specialized tissues, play a role as biological filters and modulate growth-factor activities, regulate collagen fibrillogenesis and skin tensile strength, affect tumor cell growth and invasion, and influence corneal transparency and neurite outgrowth. Additional roles, derived from studies of mutant animals, indicate that certain proteoglycans are essential to life whereas others might be redundant. The review focuses on the most recent genetic and molecular biological studies of the matrix proteoglycans, broadly defined as proteoglycans secreted into the pericellular matrix. Special emphasis is placed on the molecular organization of the protein core, the utilization of protein modules, the gene structure and transcriptional control, and the functional roles of the various proteoglycans. When possible, proteoglycans have been grouped into distinct gene families and subfamilies offering a simplified nomenclature based on their protein core design. The structure-function relationship of some paradigmatic proteoglycans is discussed in depth and novel aspects of their biology are examined.

The Collagen Family

Abstract: Collagens are the most abundant proteins in mammals. The collagen family comprises 28 members that contain at least one triple-helical domain. Collagens are deposited in the extracellular matrix where most of them form supramolecular assemblies. Four collagens are type II membrane proteins that also exist in a soluble form released from the cell surface by shedding. Collagens play structural roles and contribute to mechanical properties, organization, and shape of tissues. They interact with cells via several receptor families and regulate their proliferation, migration, and differentiation. Some collagens have a restricted tissue distribution and hence specific biological functions.

Role of Extracellular Matrix in Adaptation of Tendon and Skeletal Muscle to Mechanical Loading

Abstract: The extracellular matrix (ECM), and especially the connective tissue with its collagen, links tissues of the body together and plays an important role in the force transmission and tissue structure maintenance especially in tendons, ligaments, bone, and muscle. The ECM turnover is influenced by physical activity, and both collagen synthesis and degrading metalloprotease enzymes increase with mechanical loading. Both transcription and posttranslational modifications, as well as local and systemic release of growth factors, are enhanced following exercise. For tendons, metabolic activity, circulatory responses, and collagen turnover are demonstrated to be more pronounced in humans than hitherto thought. Conversely, inactivity markedly decreases collagen turnover in both tendon and muscle. Chronic loading in the form of physical training leads both to increased collagen turnover as well as, dependent on the type of collagen in question, some degree of net collagen synthesis. These changes will modify the mechanical properties and the viscoelastic characteristics of the tissue, decrease its stress, and likely make it more load resistant. Cross-linking in connective tissue involves an intimate, enzymatical interplay between collagen synthesis and ECM proteoglycan components during growth and maturation and influences the collagen-derived functional properties of the tissue. With aging, glycation contributes to additional cross-linking which modifies tissue stiffness. Physiological signaling pathways from mechanical loading to changes in ECM most likely involve feedback signaling that results in rapid alterations in the mechanical properties of the ECM. In developing skeletal muscle, an important interplay between muscle cells and the ECM is present, and some evidence from adult human muscle suggests common signaling pathways to stimulate contractile and ECM components. Unaccostumed overloading responses suggest an important role of ECM in the adaptation of myofibrillar structures in adult muscle. Development of overuse injury in tendons involve morphological and biochemical changes including altered collagen typing and fibril size, hypervascularization zones, accumulation of nociceptive substances, and impaired collagen degradation activity. Counteracting these phenomena requires adjusted loading rather than absence of loading in the form of immobilization. Full understanding of these physiological processes will provide the physiological basis for understanding of tissue overloading and injury seen in both tendons and muscle with repetitive work and leisure time physical activity.